Expanding waistlines are a major mid-life bugbear for many people as that expensive wedding outfit gets a bit tighter each time.

Now, scientists believe they have pinpointed the primary culprit - and it's not the dry cleaner.

They have found that the average person gains 20 per cent in weight during their adult life, assuming food intake, exercise and other weight-related factors remain constant.

The weight gain, which can be offset by eating less and exercising more, occurs because the fat cells becomes less effective at removing accumulated fat, a new study finds.

It's all about the fat cells

Fat that isn't burned up for energy shortly after consumption makes its way to the fat cells.

These gradually break it down into fatty acids and glycerol, which are released into the bloodstream - where they get broken down further and are eventually released from the body through a person's urine or breath.

But the process gets slower as people get older, meaning that the amount of fat residing in the fat cells gets bigger.

The researchers said their findings could pave the way for weight loss drugs further down the line.

"The results indicate for the first time that processes in our fat tissue regulate body weight during ageing....This could open up new ways to treat obesity," said Professor Peter Arner, of the Karolinska Institutet in Sweden.

What happens next

The team will now work to identify the genes involved in regulating the rate at which fat - or lipids - are processed.

The amount of weight that people gain weight as a result of the fat-processing slowdown during adulthood varies.

The point at which the slowdown begins - and its duration - also differs from one person to the next, researchers said.

However, on average, the change takes place over a 13 year period and involves a 20 per cent weight gain, the study found.

Obesity is a major health issue

Kirsty Spalding, also of the Karolinska Institute, said: "Obesity and obesity-related diseases have become a global problem. Understanding lipid dynamics and what regulates the size of the fat mass in humans has never been more relevant."

The study is published in the journal Nature Medicine and also involved Uppsala University in Sweden and the University of Lyon in France.

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